1. Field of the Invention
The invention relates to a high-intensity discharge lamp and a lighting device.
2. Description of Related Art
In addition to being used to light open spaces such as roads, plazas, stadiums and the like, and as the light source for shops and vehicles, high-intensity discharge lamps, for example metal halide discharge lamps, are also widely used as the light source for optical devices such as overhead projectors and liquid crystal projectors.
A metal halide lamp is a discharge lamp having an electrode structure and an arc tube filled with a discharge medium. The discharge medium generally includes a metal halide, mercury or a noble gas. The atomic spectral lines or the molecular spectrum of the filled metal halide are used as the source of emission to provide a lamp with higher luminous efficacy, higher correlated color temperature and higher color rendition than a mercury lamp.
Halides such as metal iodides or metal bromides, used as the source of emission for this metal halide lamp, in addition to mercury, and including metals such as Na, In, Tl, Li and Cs, or rare earth metals such as Dy, Ho, Tm, Sc, Nd, Ce, fill the arc tube, thereby creating a structure which offers good emission properties.
However, it may be difficult to develop a single discharge lamp with excellent values over a range of characteristics, such as efficiency or correlated color temperature. Thus, rendition and life due to failings like color rendition may be poor despite high efficiency. Conversely, efficiency may still be poor despite good color rendition, or may vary with operating position of the lamp.
In recent years, metal halide lamps with improved efficiency, correlated color temperature, and color rendition and life have been obtained. These lamps generally include a compact arc tube having translucent ceramic materials. Translucent ceramic materials have superior resistance to corrosion and heat, and react less with metal halides than does quartz glass,
The application of such metal halide lamps with filled metal halides has expanded, although they have not been much used to provide light in various directions. Thus, if this type of lamp is not changed with the operating position, the lamp may fade or have a shortened life due to great variations in its properties and reduction in efficiency or color variation on the illuminated surface with changes in operating position.
JP-PS 3293499 discloses a high-intensity discharge lamp with metal halides, including rare earth metal halides and sodium halide, filled within an arc tube including a translucent ceramic chamber. In this reference, the sodium halide is mixed in at a relative weight of 10–100% with respect to the quantity of rare earth metal halides (DyI 55 Wt %: NaI 30 Wt %: TlI 15 Wt %). JP-PS 3293499 discloses that, in addition to delivering excellent emission characteristics having a luminous efficacy of 96 Lm/W, a correlated color temperature of 4100K (3500–5000K), and an average color rendition index value (Ra) of 95, this lamp offers a low extinguish voltage difference between vertical operation and horizontal operation.
However, when measuring the characteristics of a lamp disclosed in JP-PS 3293499, the desired emission characteristics were not always obtained from lamps with a power rating different from the standard power rating cited in the embodiments of Reference 1.
JP-PS 2003-16998 does not disclose the dimensions that can be used to determine the evaporation temperature (coldest point) of the metal halides filled within the arc tube, or the dimensions of the structure of the lamp. In addition, the cited characteristics were not obtained for some varieties of these selected rare earth halides.
JP-PS 2003-16998 discloses a lamp having high luminous efficacy (117 Lm/W and above). This reference discloses that control of the lumen maintenance factor is achieved with a metal halide lamp including an arc tube and a translucent ceramic chamber filled with a mixture (100 wt % in total) of cerium halide (20–69 wt %), sodium halide (30–79 wt %), thallium halide and indium halide (Tl and In halides comprising a combined weight of 1–20 wt %).
However, while high luminous efficacy and control of the lumen maintenance factor may be provided with lamps constructed according to the specifications of JP-PS 2003-16998, in addition to the light emitted from the lamp being very green, the average rendition index values generally fall to 75 and below. This means that this lamp may not be suited for application in retail outlets for lighting outdoor spaces.
JP-PS 7-130331 discloses, a lamp where color temperature stability and good color rendition are done by adjusting the ratio of the filled metal halides. In this reference, a lamp with a power rating of 30–40 W has a tube wall loading of 20–26 W/cm2 and a color temperature of 2800–3700K.
JP-PS 62-66556 discloses a metal halide lamp having an arc tube and a translucent ceramic chamber which radiates light with good luminous efficacy (103 Lm/W and above), correlated color temperature (3600K), average color rendition index value (Ra=87) and chromacity coordinates (x=0.401, y=0.395). The arc tube of the lamp is filled with a mixture (100 wt % in total) of thulium halide (16 wt %), sodium halide (77 wt %) and thallium halide (7 wt %).
However, experiments have shown that the light emitted during the life of the lamp, disclosed in JP-PS 62-66556, was extremely red and that the variation in correlated color temperature with lighting direction (vertical lighting and horizontal lighting) was 500 K and more. Furthermore, it has been noted that an increase in the average color rendition index values over the life of the lamp led to a deterioration in luminous efficacy, and a dramatic decrease in the lumen maintenance factor. This may be due to the relative low weight of Tm halide (16 wt %) and the relative high weight of the sodium halide.